Method and System for Treating Ultrapure Water

ABSTRACT

According to various aspects and embodiments, a system and method for polishing ultrapure water (UPW) is disclosed. The water polishing system includes a source of ultrapure water (UPW), an ultrafiltration (UF) module having an inlet and a permeate outlet, a recirculation conduit communicating the permeate outlet with the inlet and forming a recirculation loop, a recirculation pump disposed along the recirculation conduit upstream from the inlet of the UF module and fluidly coupled to the source of UPW, a supply conduit fluidly coupled to the recirculating conduit and a demand source, the supply conduit positioned downstream from the permeate outlet, and a pressure control valve disposed along the recirculation conduit downstream from the supply conduit and configured to maintain pressure of permeate at a predetermined value.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Application Ser. No. 62/394,788, titled “POLISHINGULTRAFILTRATION PERFORMANCE ENHANCEMENT FOR VANOX POU, POU FILTRATION,HWC AND FULL FLOW POLISHING UF,” filed on Sep. 15, 2016, and to U.S.Provisional Application Ser. No. 62/435,119, titled “FILTRATION SYSTEMFOR INDUSTRIAL WATER TREATMENT,” filed on Dec. 16, 2016, each of whichis herein incorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates generally to water treatment and, moreparticularly, to a water polishing system for ultrapure water (UPW).

BACKGROUND

Advances in semiconductor design and manufacturing have led to thedevelopment of increasingly smaller and more powerful devices. Thesmaller dimensions used in these structures makes them increasinglysensitive to the presence of smaller particles present in processingfluids used for manufacturing the devices, such as UPW. Particles of 20nm and smaller may significantly impact the yield of certainsemiconductor devices. The detection and removal of these smallparticles has proven to be a difficult problem to resolve.

SUMMARY

Aspects and embodiments are directed to filtration for particle controlin ultrapure water.

According to one embodiment a water polishing system includes a sourceof ultrapure water (UPW), an ultrafiltration (UF) module having an inletand a permeate outlet, a recirculation conduit communicating thepermeate outlet with the inlet and forming a recirculation loop, arecirculation pump disposed along the recirculation conduit upstreamfrom the inlet of the UF module and fluidly coupled to the source ofUPW, a supply conduit fluidly coupled to the recirculating conduit and ademand source, the supply conduit positioned downstream from thepermeate outlet, and a pressure control valve disposed along therecirculation conduit downstream from the supply conduit and configuredto maintain a pressure of permeate at a predetermined value.

In one example the water polishing system further includes a pressuresensor positioned along the recirculation conduit between the supplyconduit and the pressure control valve, where the pressure sensor isconfigured to generate a pressure signal indicative of a pressure of thepermeate. In one example the water polishing system further includes acontroller operatively coupled to the pressure sensor and configured tocontrol operation of the pressure control valve based on the pressuresignal.

In one example the water polishing system further includes a UPW make-upconduit fluidly coupled to the source of UPW and the recirculationconduit, and a blending point where the UPW make-up conduit fluidlyconnects to the recirculation conduit such that the pressure controlvalve is positioned between the blending point and the pressure sensor,and the controller is configured to operate the pressure control valvesuch that the pressure of the permeate is maintained at a predeterminedpressure value.

In one example the recirculation pump is positioned downstream from theblending point such that a fluid including UPW and permeate is deliveredto the recirculation pump at the predetermined pressure value.

In one example the recirculation pump is configured to operate at apredetermined constant speed.

In one example the UF module further includes a reject outlet forretentate from the UF module and a reject conduit fluidly coupled to thereject outlet.

In one example the water polishing system further includes a holdingtank disposed along the recirculation conduit.

In one example the water polishing system further includes a vibrationsource coupled to the UF module.

Another embodiment is directed to a method of treating water. The methodincludes receiving a flow of ultrapure water (UPW) directed toward arecirculation pump from a source of UPW, setting the speed of therecirculation pump to a predetermined constant speed, directing a fluidincluding the UPW to an inlet of an ultrafiltration (UF) module,recirculating at least a portion of permeate from a permeate outlet ofthe UF module to the inlet using the recirculation pump and arecirculation conduit, providing a valve arrangement actuable from aclosed to an open condition for effecting pressure of permeate to therecirculation pump, measuring a pressure of the permeate at a positionthat is upstream from the valve arrangement and downstream from thepermeate outlet, comparing the measured pressure of the permeate to apredetermined value, and responsive to the comparison, actuating thevalve arrangement.

In one example the valve arrangement is actuated to maintain asubstantially constant transmembrane pressure (TMP) across a UF membraneof the UF module.

In one example the method further includes combining the flow of UPWwith permeate at a blending point positioned along the recirculationconduit such that the valve arrangement is positioned upstream from theblending point.

In another example the method further includes providing a supplyconduit to a demand source along the recirculation conduit at a positionupstream from the pressure measurement position and downstream from thepermeate outlet.

In another example the recirculation pump is configured to deliver thefluid to the UF module at a pressure that is greater than thepredetermined value.

In one example the method further includes applying a vibration to theUF module. Another embodiment is direct to a method of facilitatingpolishing of ultrapure water (UPW). The method includes providing anultrafiltration (UF) module having an inlet and a permeate outlet, andproviding instructions to: connect the permeate outlet to the inlet,connect the permeate outlet to a pressure control valve, and maintain apressure of permeate exiting the permeate outlet at a predeterminedvalue with the pressure control valve.

In one example, the method further includes providing instructions to:connect a recirculation pump to the inlet of the UF module, blend thepermeate with a source of UPW to form a fluid, and direct the fluid toan inlet of the recirculation pump.

In one example, the method further includes providing instructions to:connect the permeate outlet to a demand source, and measure a pressureof the permeate using a pressure sensor prior to blending with thesource of UPW.

In one example the method further includes providing a controller thatis configured to be operatively coupled to the pressure sensor and thepressure control valve. In another example the method further includesproviding control instructions to the controller to compare a measuredpressure of the permeate to a predetermined value and, responsive to thecomparison, actuate the pressure control valve. In one example thecontrol instructions instruct the controller to actuate the pressurecontrol valve to maintain a substantially constant transmembranepressure (TMP) across a UF membrane of the UF module. In one example,the method further includes providing at least one of the recirculationpump and the pressure control valve.

Still other aspects, embodiments, and advantages of these exampleaspects and embodiments, are discussed in detail below. Moreover, it isto be understood that both the foregoing information and the followingdetailed description are merely illustrative examples of various aspectsand embodiments, and are intended to provide an overview or frameworkfor understanding the nature and character of the claimed aspects andembodiments. Embodiments disclosed herein may be combined with otherembodiments, and references to “an embodiment,” “an example,” “someembodiments,” “some examples,” “an alternate embodiment,” “variousembodiments,” “one embodiment,” “at least one embodiment,” “this andother embodiments,” “certain embodiments,” or the like are notnecessarily mutually exclusive and are intended to indicate that aparticular feature, structure, or characteristic described may beincluded in at least one embodiment. The appearances of such termsherein are not necessarily all referring to the same embodiment.

BRIEF DESCRIPTION OF DRAWINGS

Various aspects of at least one embodiment are discussed below withreference to the accompanying figures, which are not intended to bedrawn to scale. The figures are included to provide an illustration anda further understanding of the various aspects and embodiments, and areincorporated in and constitute a part of this specification, but are notintended as a definition of the limits of any particular embodiment. Thedrawings, together with the remainder of the specification, serve toexplain principles and operations of the described and claimed aspectsand embodiments. In the figures, each identical or nearly identicalcomponent that is illustrated in various figures is represented by alike numeral. For purposes of clarity, not every component may belabeled in every figure. In the figures:

FIG. 1 is a graph showing the percent retention efficiency for afiltration system in removing particles of various sizes from UPW;

FIG. 2 is a graph showing the concentration of particles in permeatefrom a filtration unit during a flow disturbance event;

FIG. 3 is a schematic flow diagram of a prior art point of use (POU)water treatment system;

FIG. 4 is a schematic flow diagram of one example of a water polishingsystem in accordance with one or more aspects of the invention;

FIG. 5 is a schematic diagram of a water polishing system in accordancewith one or more aspects of the invention; and

FIG. 6 is a schematic flow diagram of another example of a waterpolishing system in accordance with one or more aspects of theinvention;

FIG. 7 is a graph showing the temperature of permeate at various flowrates;

FIG. 8 is a graph showing particle concentration over time;

FIG. 9 is a first graph showing accelerometer data over time;

FIG. 10 is a second graph showing accelerometer data over time; and

FIG. 11 is a graph showing the percent retention efficiency for anultrafiltration system in removing particles of various sizes from UPWin accordance with certain aspects of the invention.

DETAILED DESCRIPTION

The aspects disclosed herein in accordance with the present invention,are not limited in their application to the details of construction andthe arrangement of components set forth in the following description orillustrated in the accompanying drawings. These aspects are capable ofassuming other embodiments and of being practiced or of being carriedout in various ways. Examples of specific implementations are providedherein for illustrative purposes only and are not intended to belimiting. In particular, acts, components, elements, and featuresdiscussed in connection with any one or more embodiments are notintended to be excluded from a similar role in any other embodiments.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. Any references toexamples, embodiments, components, elements or acts of the systems andmethods herein referred to in the singular may also embrace embodimentsincluding a plurality, and any references in plural to any embodiment,component, element or act herein may also embrace embodiments includingonly a singularity. References in the singular or plural form are notintended to limit the presently disclosed systems or methods, theircomponents, acts, or elements. The use herein of “including,”“comprising,” “having,” “containing,” “involving,” and variationsthereof is meant to encompass the items listed thereafter andequivalents thereof as well as additional items. References to “or” maybe construed as inclusive so that any terms described using “or” mayindicate any of a single, more than one, and all of the described terms.In addition, in the event of inconsistent usages of terms between thisdocument and documents incorporated herein by reference, the term usagein the incorporated reference is supplementary to that of this document;for irreconcilable inconsistencies, the term usage in this documentcontrols.

As discussed above, there is presently a need for the ability to removeparticles having a diameter of 20 nm and smaller from semiconductordevice fabrication processes, including process that use UPW. Certaincurrent applications and future semiconductor devices will require theability to remove particles sized at 10 nm and smaller.

Typical UPW delivery systems suffer from a number of deficiencies inmeeting these requirements. For instance, the filters that are used maynot be able to completely remove particles sized at 20 nm or smaller.The data presented in the graph of FIG. 1 shows the percent retention ofa cartridge filtration device according to particle size. As indicatedin FIG. 1, the cartridge filter was effective at removing over 95% ofparticles having a diameter larger than 25 nm, but was only capable ofremoving between about 67-87% of particles sized at or between 10-20 nm.

Another deficiency of typical UPW delivery systems is that the filtersmay produce particles when they are operated under varying flow rate andpressure conditions. The graph presented in FIG. 2 shows theconcentration of particles in the filtrate of a cartridge filtrationdevice over time when the system was challenged with both a colloidalmaterial having a particle concentration of 5×10⁹ particles permilliliter and a flow disturbance, i.e., flow rate and pressurefluctuation. The results show that the flow disturbance caused a spikein the concentration of particles in the effluent to a value thatexceeded 1×10⁹ particles per milliliter, which indicates that flowfluctuations can drive particles through the filter. Lastly, yet anotherdeficiency of typical UPW delivery systems is that the fibers used inthe membranes of the UF devices are susceptible to flow and pressurevariations, as well as oxidants, both of which can cause fiber breakage.

Referring to FIG. 3, a schematic diagram of one example of a typicalpoint of use (POU) water purification system is shown and generallydesignated by reference number 10. The system 10 includes a filtrationunit 20 positioned downstream from a source of UPW 105 and is configuredfor operation in a single-pass mode, such that the UPW passes oncethrough the filtration unit 20. As used herein, the term “point of use”refers to a filtration system that is located in proximity to a demandsource 150, such as a semiconductor manufacturing tool. In certaininstances, the demand source 150 may be positioned in a sub-fab orutility level location positioned under the tool. The filtration unit 20shown in the POU system 10 of FIG. 3 is a cartridge filter that isconfigured for dead-end filtration. In dead-end filtration feed (theultrapure water from the UPW source 105) is passed through a filter suchas a membrane or bed where solids are trapped or otherwise retained bythe filter, and permeate 15 exits the filtration unit 20.

The POU system 10 is supplied with UPW 105 from a main UPW treatmentsystem. Many industrial applications of water, including semiconductorfabrication, require the use of UPW, which is water that has a very loworganic carbon content, i.e., a total organic carbon (TOC) level that isless than about 25 ppb, and in some instances may be less than about 1ppb. Total organic carbon refers to the parts per billion of water thatare carbon atoms associated with organic compounds. UPW may be usedduring various steps of the manufacturing process to rinse variouschemicals and particulate materials from the devices, such as bydissolving organic deposits from substrate surfaces. The UPW must meetstringent quality requirements since contaminants in the UPW can have anegative effect on the yield of the semiconductor devices. Typicalcontaminants include organic and inorganic compounds, as well asparticulate matter.

The UPW treatment system is equipped with one or more devices thatgenerate the UPW and can include, for example, processes that includefiltration, ion exchange, and exposure to ultraviolet radiation. Forinstance, the UPW treatment system may include pressure driven membraneprocesses such as reverse osmosis and nanofiltration, deionizationprocesses such as electrodeionization and regenerable ion exchange, andparticulate removal processes such as ultrafiltration and sub-micronparticle filters. Another example of a UPW treatment system that may beused for providing the source of UPW 105 is a system that uses anadvanced oxidation process (AOP). Advanced oxidation processes generallyrefer to oxidation methods that are based on generation of strong andnon-selective free radicals, which attack and destroy organiccontaminants in the water. Non-limiting examples of oxidizing agentsinclude ultraviolet light, ozone, hydrogen peroxide, and persulfate.Contaminated water reacts with one or more oxidizing agents, such ashydrogen peroxide combined with ultraviolet light to generate thereactive radicals. The organic carbon is thereby oxidized to carbondioxide, which becomes dissolved in water. One example of a UPW systemthat may be used to generate the source of UPW 105 is the VANOX® AOPSystem available from Evoqua Water Technologies (Pittsburgh, Pa.).

According to certain embodiments, the UPW 105 generated by the UPWtreatment system includes water that has a TOC that is less than 5 ppb.In some instances the source of UPW may have a TOC concentration of lessthan 1 ppb. In addition, the UPW 105 may have an electrical resistivityin a range of between about 17-18.2 megaohm-cm, and in some instancesmay have an electrical resistivity of about 18.2 megaohm-cm. Theparticle concentration (particles sized at 50-100 nm) of the UPW 105 maybe less than 200 particles per liter. The UPW 105 may also have one ormore other properties, including a dissolved oxygen of less than 3 ppb,an on-line residue of less than 100 ppt, a bacteria count that is lessthan 1 CFR/100 mL sample, and a total silica concentration of less than1 ppb.

The source of UPW 105 may be delivered to the POU system 10 at apredetermined pressure, which may be a value that exceeds the minimumpressure required by the demand source 150. In some instances, thepressure exerted by the UPW 105 in the POU system 10 of FIG. 3 functionsas the driving force for passing the UPW through the filtration unit 20.In some instances, the UPW 105 may be at a pressure ranging from 3-5bars (˜43.5-72.5 psi). In some embodiments, the UPW 105 may be at apressure greater than 5 bars.

The POU system 10 of FIG. 3 is configured to operate in the on-offregime using the valve 30 according to the consumption requirements ofthe demand source 150. Therefore, the only time the filtration unit 20treats the UPW 105 is when the demand source 150 actually consumespermeate 15. In addition, the filtration unit 20 will only treat the UPW105 at the flow rate that the demand source 150 consumes the permeate15. This type of intermittent flow through the filtration unit 20creates the risk of particles being released from the filter. This isbecause the pulse created by the sudden increase in the fluid flow rategenerates an additional driving force for particles sized smaller thanthe filter pores to escape the filter, i.e., breakthrough, and can alsogenerates particles that originate from the filter material itself.

As noted above, the UPW treatment system may provide UPW 105 at apredetermined pressure that exceeds the minimum pressure required by thedemand source 150. However, when the head loss in the filtration unit 20is greater than the minimum pressure required by the demand source 150,then the filtration unit 20 can no longer be used. Furthermore, as themechanical strength of the filter increases and pore size decreases, thehead loss further increases. These constraints make it increasinglydifficult to use the POU system 10 for providing ultrapure water thatmeets modern or future requirements for particulates.

In accordance with at least one embodiment, a water treatment system isprovided that is capable of providing POU ultrapure water to asemiconductor process. The water treatment system may function as awater polishing system that is positioned downstream from a source ofUPW and removes or substantially reduces the presence of particles thatare 20 nm, and in some instances, 10 nm or smaller in diameter fromultrapure water delivered to a demand source such as a semiconductortool.

FIG. 4 is a flow diagram of one embodiment of a water polishing system100 (also referred to herein as simply “system 100”). The system 100comprises a source of UPW 105, a UF module 110, a recirculation conduit120, a supply conduit 145, and a pressure control valve 125. The waterpolishing system 100 functions to polish UPW from a main UPW treatmentsystem and deliver the polished UPW as permeate to the demand source150.

The source of UPW 105 may be the same source as discussed above inreference to POU system 10 discussed above. For instance, the source ofUPW 105 may be generated by a main UPW treatment system, such as an AOPtreatment system, that is positioned upstream from the water polishingsystem 100. The source of UPW 105 may be provided at a certain pressure.For instance, the source of UPW 105 may be at a pressure of about 20 psito about 100 psi. The pressure of the UPW provided by the UPW source 105may depend on the pressure requirements of the demand source 150. In oneembodiment, the UPW 105 is at a pressure in a range of about 3-5 bar. Inother embodiments, the UPW 105 is at a pressure that is greater than 5bar. The source of UPW 105 may also be provided at a certain flow rate.For example, in some instances the source of UPW may be at a flow rateof about 0 to about 50 gpm. According to one embodiment, the source ofUPW may have a flow rate of 20-30 gpm.

The UF module 110 functions as a filtration device for removingundesirable contaminants, such as particulates, including particulateshaving high molecular weights and particulates having a certain size.The UF module 110 has an inlet 114 for receiving fluid 170 and apermeate outlet 116 (labeled as 116 a and 116 b in FIG. 6) for permeate126. Although not explicitly shown, the UF module 110 also includes atleast one UF membrane that provides the filtration functionality. The UFmembrane may be arranged according to a hollow fiber design as known inthe art, and may be dependent on a particular application and/or user'spreferences. According to one embodiment, the UF membrane has amolecular cut off (MWCO) of at least 4000. According to someembodiments, the UF membrane has a MWCO of 6000-10,0000. It isrecognized that future advances in UF membrane technology may bedeveloped and would be applied in the process and system identified inthis disclosure. Although the UF module 110 shown in FIG. 4 includes twopermeate outlets, UF modules having a single permeate outlet are alsowithin the scope of this disclosure.

In contrast to the dead-end filtration exemplified in POU system 10 ofFIG. 3 where the flow of feed solution being perpendicular to themembrane surface, the UF module 110 of the water polishing system 100 ofFIG. 4 may be configured as a cross-flow filtration device, where themajority of the feed flow travels tangentially across the UF membrane,i.e., parallel to the membrane surface, at a positive pressure relativeto the permeate side, and therefore the separation process is driven bya pressure gradient across the UF membrane. The cross-flow configurationof the UF module 110 also allows for continuous flow such that solidsare continuously flushed from the membrane surface. A proportion of thematerial that is smaller than the membrane design retention size passesthrough the membrane as permeate, while the rest is rejected asretentate 128 (otherwise referred to herein as reject water) out areject outlet 122 of the UF module 110. A reject conduit 124 is fluidlycoupled to the reject outlet 122. Non-limiting examples of a UF modulethat may be used includes the MICROZA™ OLT-6036 ultrafiltration systemavailable from Asahi Kasei Corporation and the VANOX® POU-F Systemavailable from Evoqua Water Technologies.

Although the examples discussed herein include an ultrafiltration deviceas the filtration mechanism in the water polishing system 100, othertypes of filters are also within the scope of this disclosure. Forinstance, microfilters, nanofilters, or reverse osmosis devices may alsobe used in accordance with certain aspects.

The water polishing system 100 also includes a recirculation conduit 120that is configured to communicate the permeate outlet 116 of the UFmodule 110 with the inlet 114 and forms a recirculation loop such thatpermeate is continuously recirculated through the UF module 110. Unlikethe system 10 of FIG. 3, flow through the filter (e.g., the UF module110) is constant and is independent of consumption by the demand source150. The water polishing system 100 also includes a recirculation pump115 that is disposed along the recirculation conduit 120 upstream fromthe inlet 114 of the UF module 110. The recirculation pump 115 isfluidly coupled to the source of UPW 105. The recirculation pump 115pumps a fluid 170 that includes a blend of both UPW 105 and permeate 126to the inlet 114 of the UF module 110. The recirculation pump 115 isconfigured to help provide a driving force for moving fluid through therecirculation conduit 120 by providing additional pressure to the fluid170. This additional pressure is needed so that permeate 126 can bedelivered to the demand source 150 at the minimum pressure required bythe demand source 150 after passing through the UF module 110 and othercomponents of the system that may reduce the pressure of the permeate126. According to some embodiments, the recirculation pump 115 addspressure energy to the fluid 170, thereby increasing the pressure andflow rate of fluid 170 that passes through the pump. Filters that aremechanically strong and/or have smaller pore sizes can therefore be usedin the polishing system 100 because the recirculation pump 115 canovercome the higher head losses associated with these types ofmembranes. The recirculation pump 115 may also contribute to achieving aconstant flow rate of fluid through the UF module 110. For instance, asdescribed below, the recirculation pump 115 may be set to apredetermined speed that remains constant throughout a filtrationprocess. The pressure of the fluid exiting the recirculation pump 115may be dictated by characteristics of the pump as well as one or morecomponents positioned downstream of the pump, including valves. In someembodiments, the recirculation pump 115 is configured as a centrifugaltype of pump as known in the art. For these types of pumps, thedischarge pressure increases with the square of the pump speed. Thepresence of the recirculation pump 115 in system 100 can thereforeeliminate or substantially reduce the problems associated withintermittent flow created by the system 10 of FIG. 3, e.g., particlegeneration.

According to one embodiment, a reject valve 142 positioned on the rejectconduit 124 may be used to create a backpressure that provides thedriving force for permeate to flow through the UF module 110.

In accordance with certain embodiments, the recirculation pump 115 maybe equipped with an adjustable speed drive system which can becontrolled by a controller (discussed further below). For example, thespeed of the recirculation pump 115 may be adjustable from zero to 8000rpm. Other ranges of speed are also within the scope of this disclosure,and may be dependent on a particular application and system design. Thevariable speed motor driving the recirculation pump 115 can be set to apredetermined constant speed by a controller (discussed further below)or by a user based on the flow characteristics required by the UFmembrane used in the UF module 110. The controller or user can thereforeset the speed of the recirculation pump 110 using the adjustable speeddrive of the pump. Increasing the speed of the motor driving therecirculation pump 115 increases the output volume of the recirculationpump 115, whereas decreasing the speed of the motor decreases the outputvolume.

According to other embodiments, the variable speed motor driving therecirculation pump 115 can be controlled in response to one or moreoperating parameters, including flow rates, temperatures, and/orpressures of fluids in the water polishing system 100. For instance, aflow rate signal may be used by a controller to control the speed of therecirculation pump 115. According to another example, a pressure signalfrom the pressure sensor 140 may be used by the controller to controlthe speed of the recirculation pump 115.

In some embodiments the recirculation pump 115 may be constructed andarranged to minimize particle generation in fluid 170. For instance, therecirculation pump 115 may be configured to minimize or otherwise applylow shear forces to the fluid 170. According to one embodiment, therecirculation pump is a bearingless magnetically levitated centrifugalpump. This type of configuration minimizes the generation of particlesthat can be abraded from moving components of the drive motor. Forinstance, magnetic levitation allows for the pump's impeller to besuspended, contact-free, inside a sealed casing where the impeller canbe driven by the magnetic field of the motor. The recirculation pump 115may also be constructed from materials that minimize particle shedding,such as fluorocarbon resins and stainless steel. Suitable examples ofrecirculation pumps that may be included in the disclosed systemsinclude centrifugal pumps such as the BPS series available fromLevitronix, LLC (Waltham, Mass.).

Referring back to FIG. 4, a supply conduit 145 is fluidly coupled to therecirculation conduit 120 and the demand source 150. The supply conduit145 is positioned downstream from the permeate outlet 116 of the UFmodule 110 and supplies permeate 126 to the demand source 150. A flowrate sensor 130 may be disposed downstream from the permeate outlet 116along the recirculation conduit 120. The flow rate sensor 130 may beconfigured to generate a flow rate signal indicative of a flow rate ofthe permeate 126.

The water polishing system 100 also includes a pressure control valve125 that is disposed along the recirculation conduit 120 downstream fromthe supply conduit 145. The pressure control valve 125, also referred toherein as a valve arrangement, may be a diaphragm type pressure controlvalve as known in the art. The pressure control valve 125 is actuablefrom a closed to an open condition for effecting pressure of permeate126 to the recirculation pump 115. As explained in further detail below,the pressure control valve 125 is configured to regulate or otherwisecontrol the pressure of the permeate 126 based on measurements from apressure sensor 140 positioned upstream from the pressure control valve125. A controller may be used to control operation of the pressurecontrol valve 125 for purposes of maintaining a pressure of the permeate126 at a predetermined value based on measurements taken from thepressure sensor 140.

A pressure sensor 140 is positioned along the recirculation conduit 120between the supply conduit 145 and the pressure control valve 125, suchthat the pressure sensor 140 is located downstream from the supplyconduit 145 and upstream from the pressure control valve 125. Thepressure sensor 140 is configured to generate a pressure signalindicative of a pressure of the permeate 126 and to send or otherwisetransmit the pressure signal to a controller, as explained in furtherdetail below.

Constant flux through the UF module 110 is independent of consumption ofpermeate 126 of the demand source 150 and is achieved by therecirculation of the permeate 126 through the recirculation conduit 120and can be maintained by the recirculation pump 115 and the pressurecontrol valve 125. According to one embodiment, flow through the UFmodule 110 is constant, and is substantially the same as or greater thanthe maximum flow required by the demand source 150. When the demandsource 150 consumption is less than the maximum flow (e.g., the tool isnot in use) then flow in excess of the actual consumption of the demandsource 150 is constantly recirculated to the suction side of therecirculation pump 115. A constant pressure may be maintained by thepressure control valve 125, which is regulated by a controller based onthe value of the pressure measurements taken by the pressure sensor 140.Both constant pressure and constant flow reduce the risk of particleshedding by the UF module 110 and protect the filter material frompotential damage.

The water polishing system 100 also includes a UPW make-up conduit 107that is fluidly coupled to the source of UPW 105 and the recirculationconduit 120. The UPW make-up conduit 107 delivers UPW from the main UPWtreatment system to the water polishing system 100 for final polishingbefore being delivered to the demand source 150. The make-up conduit 107may also include one or more valves, such as valve 118, for isolatingthe flow of the UPW to the recirculation conduit 120. For instance,during periods of maintenance, the valve 118 may be used to shut off thesource of UPW 105 to isolate the rest of the system. The make-up conduit107 and recirculation conduit 120 are configured to direct the UPWtoward the recirculation pump 115, i.e., the inlet or suction side ofthe recirculation pump 115, as indicated in FIG. 4.

The water polishing system also includes a blending point 147 where theUPW make-up conduit 107 fluidly connects to the recirculation conduit120. The blending point 147 allows for permeate 126 from the UF module110 to blend or otherwise combine with UPW from the source of UPW 105 asfluid 170, which is a blended feed flow to the recirculation pump 115and inlet 114. The blending point 147 is positioned in the system suchthat the pressure control valve 125 is located between the blendingpoint 147 and the pressure sensor 140. As discussed in further detailbelow, a controller is configured to operate the pressure control valve125 such that the pressure of the permeate 126 is maintained at apredetermined pressure value. The recirculation pump 115 is positioneddownstream from the blending point 147 such that fluid 170 that includesUPW 105 and permeate 126 is delivered to the recirculation pump 115.

The water polishing system 100 may also include at least one temperaturesensor 135. In some instances the flow rate of the permeate 126 isdirectly proportional to the water temperature. Thus, if watertemperature increases, then the flow rate of the permeate will alsoincrease (assuming a constant reject flow rate). During periods wherethere is no demand from the demand source 150, the temperature of thepermeate will increase by a predetermined amount (e.g., 1.2-2.0° C. or0.9-1.6° C.) as compared to the temperature of the incoming UPW 105(assuming a constant reject water flow rate). At higher flow rates, thetemperature increase is less. Assuming the transmembrane pressure (TMP)is constant (which, as discussed below, is maintained at a substantiallyconstant value using the pressure sensor 140 and the pressure controlvalve 126), the temperature increase can result in an increase in theflow rate of the permeate. For example, in some instances the increasein temperature results in a 3-6% increase in the flow rate of thepermeate.

In the embodiment shown in FIG. 4 a temperature sensor 135 is coupled tothe reject conduit 124 and is configured to generate a temperaturesignal indicative of a temperature of the retentate 128. Placement ofthe temperature sensor 135 on the reject conduit 124 reduces the numberof fittings and obstructions in the recirculation conduit 120, whichavoids particle generation. Furthermore, the temperature of the permeate126 is the same as the retentate 128. A controller, as described furtherbelow, may be configured to control one or more components of the systembased on the temperature signal. For instance, the controller may beconfigured to change a speed of the recirculation pump 115 and/or adjustthe pressure control valve 125 based on the temperature signal. In suchinstances, the temperature sensor 135 sends a temperature signalindicative of the temperature of the retentate 128 to the controller160. The controller 160 may then change a speed of the recirculationpump 115 and/or adjust the pressure control valve 125 based on thetemperature signal. For example, the demand source 150 may requirepermeate 126 to be within a certain range of predetermined temperaturevalues. The controller 160 may therefore use the temperature readingsfrom the temperature sensor to control the pressure control valve 125and/or recirculation pump 115 or other components of the system inresponse to the temperature measurement for purposes of bringing thetemperature of the permeate into compliance with the range ofpredetermined temperature values.

According to one embodiment, the water polishing system 100 furtherincludes a vibration source 165. The vibration source 165 may be coupledto the UF module 110. For instance, the vibration source 165 may beattached to an external wall or another external element of the UFmodule 110, where the vibrational energy may then be transferred to theUF membrane. The vibration source 165 is configured to deliver a lowlevel vibration to the UF module 110 which results in the removal ofsmall particles from the UPW being filtered. For example, the vibrationsource 165 may remove particles sized at 30 nm and smaller, 20 nm andsmaller, and in some instances 10 nm and smaller from the UPW beingfiltered. Applying a vibration source to the UF module causes the UFmembrane to retain these smaller-sized particles. According to someembodiments, the vibration source may be applied intermittently atpredetermined intervals during a filtration process. Application of thevibration source may also be a function of membrane fouling. Forinstance, the vibration source may be applied at the beginning of afiltration process (i.e., after the membrane has undergone a cleaningprocess or has just been installed), middle, or end of the process(i.e., after a duration of use, but prior to the membrane beingcleaned).

The vibration source 165 may be any device capable of delivering a lowlevel mechanical vibration. The vibration source 165 may be a deviceconfigured as a mechanical oscillator that is capable of deliveredmechanical energy in the form of vibration to the UF membrane of the UFmodule 110. The frequency and/or amplitude of the vibration applied bythe vibration source 165 may depend on the surface area of the membrane.The vibration applied by the vibration source 165 may also beadjustable. According to some embodiments, a frequency range of thevibrations may be about 20 Hz to about 1000 Hz. In some embodiments, thefrequency of the vibration may be at least 60 Hz. According to certainembodiments, an amplitude range of the vibrations may be about 1 m/s² toabout 50 m/s².

According to various aspects the vibration source 165 may be used withother types of filtration units that may be included in or used inconjunction with the water polishing system 100. For example, aconditioning system may be used for preparing the UF module 110 for use.A vibration source may be attached to one or more components of theconditioning system, such as a filtration unit or platform, and avibration may be applied during at least a portion of the conditioningprocess. One example of such a conditioning system is the VANOX® SystemHot Water Conditioning (HWC) cart available from Evoqua WaterTechnologies.

Referring now to FIG. 5, in accordance with one or more embodiments, thewater polishing system 100 may include a controller 160, which isoperatively coupled to one or more components of the system 100.According to one embodiment, the controller 160 is operatively coupledto the pressure sensor 140 and the pressure control valve 125. In someembodiments, the controller 160 may also be operatively coupled to theflow rate sensor 130 and the temperature sensor 135. As described above,each of the sensors may be configured to measure a property of thesystem and to transmit or otherwise send these measurements to thecontroller 160. The controller 160 may also be operatively coupled toother components of the system 100, such as the recirculation pump 115,and in some instances the UF module 110, and may be used to controlthese components. The controller 160 may control one or more of thesecomponents based on at least one measurement taken from one or more ofthe sensors.

According to one embodiment, the water polishing system 100 isconfigured to deliver permeate 126 on-demand to the demand source 150.In some instances the water polishing system 100 may be configured todeliver permeate 126 at a predetermined flow rate, for example, at arate of between zero and 25 gpm to the demand source 150. The waterpolishing system 100 is configured to recirculate fluid using therecirculation pump 115 through the UF module 110 and the recirculationconduit 120 such that permeate is passed through the UF module 110multiple times before it is delivered to the demand source 150. Thewater polishing system 100 is also designed to minimize variations inflow rates and to minimize the generation of particles, i.e., particleshedding. As mentioned above, maintaining a constant pressure andconstant flow rate at one or more locations in the system reduces therisk of particle shedding. In one embodiment, maintaining a constantpressure and flow rate can be accomplished via the controller 160 incombination with one or more other components of the system, such as thepressure sensor 140, and the pressure control valve 125. In otherembodiments, the flow rate sensor 130 and/or recirculation pump 115 mayalso be used in maintaining a constant pressure and flow rate.

In one embodiment, the source of UPW is delivered at a first pressure,otherwise referred to herein as the UPW supply pressure. The system 100is configured to deliver permeate 126 to the demand source 150 at thefirst pressure within a certain margin of error, e.g., about ±2 psi, andin some application may be less than ±2 psi, and may be referred toherein as a predetermined value for the pressure. In certain instancesthe predetermined pressure may be the minimum pressure required by thedemand source 150, and is therefore dictated by the requirements of thedemand source 150. Delivering permeate 126 to the demand source 150 atthe predetermined pressure is accomplished using mechanisms provided bythe components of the system. For instance, the recirculation pump 115may be configured to overcome at least a portion of the pressure lossesthrough the UF module 110 (e.g., TMP), piping (conduits), and valvesused in the water polishing system 100 by increasing the pressure offluid 170 as it passes through the recirculation pump 115. In oneembodiment, the recirculation pump 115 is operated at a fixed, i.e.,constant speed such that at least a portion of the pressure losses inthe system are overcome, and the permeate 126 is delivered to the demandsource 150 at the predetermined pressure.

As stated above, the system 100 may be configured to deliver permeate126 to the demand source 150 at a predetermined flow rate value (orrange of values). The predetermined flow rate value may be the minimumflow rate required by the demand source 150. The flow rate of thepermeate 126 is proportional to the transmembrane pressure (TMP) of theUF module 110. As discussed further below, the TMP may be controlled forpurposes of maintaining a constant flow rate.

In accordance with one embodiment, the speed of the recirculation pump115 is set to a predetermined value at the beginning of a filtrationprocess and is maintained at the predetermined value. This allows for asubstantially constant flow rate of the fluid 170 to the inlet 114 ofthe UF module 110. The speed of the recirculation pump 115 may be basedon one or more factors, including flow characteristics of the UF module110. For instance, the speed of the recirculation pump 115 may be set tocorrelate with a maximum flow rate of the UF module 110. The flow rateof the fluid 170 introduced to the inlet 114 of the UF module 110 may inthis instance be the maximum flow rate that can be accepted or otherwiseprocessed by the UF module 110. In other instances, the speed of therecirculation pump 115 may be set to correlate with a minimum flow rateof the UF module 110. The flow rate of the fluid 170 introduced to theinlet 114 of the UF module 110 may in this instance be the minimum flowrate that can be accepted or otherwise processed by the UF module 110.The minimum flow rate may correspond to a flow rate that prevents orotherwise minimizes the formation of stagnant water, which couldgenerate additional unwanted particles. According to another example,the speed of the recirculation pump 115 may also be set to a speed thatis in between the maximum and minimum flow rate requirements for the UFmodule. As noted above, operating the recirculation pump 115 at a fixedspeed reduces the risk of particle generation in the pump.

The flow rate of the permeate 126 is directly proportional to thetransmembrane pressure (TMP) of the UF membrane in the UF module 110.According to some embodiments, maintaining a constant flow rate ofpermeate 126 may be accomplished by controlling the TMP since flowthrough the UF module 110 is linearly related to the TMP. The TMP is thepressure difference between the feed pressure (pressure of fluid 170entering inlet 114) and the permeate pressure (pressure of permeate 126exiting permeate outlet 116). Since the pressure of fluid 170 enteringthe UF module 110 is maintained at a constant value by the recirculationpump 115, i.e., by maintaining a constant speed of the pump, ininstances where the demand source 150 consumes permeate 126, thepermeate pressure drops (which causes the TMP to increase), which inturn creates a higher flow rate through the UF module 110. In oneembodiment, the flow rate of permeate 126 may be maintained at aconstant value by maintaining the TMP at a predetermined value.

Since the flow rate and pressure of fluid 170 entering the UF module 110is fixed by the recirculation pump, this can be accomplished bycontrolling the backpressure of the permeate 126. For instance,according to one embodiment, the pressure control valve 125 is actuatedto maintain a substantially constant TMP across the UF membrane of theUF module 110. This can be accomplished by controlling the pressurecontrol valve 125 using measurements taken by the pressure sensor 140.As indicated in FIG. 4, the pressure control valve 125 is positioned onthe recirculation conduit 120 downstream of the supply conduit 145(which supplies permeate to the demand source 150) and upstream of theblending point 147 (where permeate 126 combines with the UPW 105 fromthe make-up conduit 107). The pressure sensor 140 is positioned justupstream of the pressure control valve 125, but downstream from thesupply conduit 145. The pressure sensor 140 measures the pressure of thepermeate 126 and sends a signal to the controller 160, which comparesthe reading to a predetermined value, and responsive to the comparison,actuates or otherwise adjusts the pressure control valve 125. Forinstance, if the pressure of the permeate 126 falls below thepredetermined value, then the controller 160 can actuate the pressurecontrol valve 125 to close a certain amount to increase the pressure ofthe permeate 126. If the pressure of the permeate 126 is above thepredetermined value, then the controller 160 can actuate the pressurecontrol valve 126 to open a certain amount to decrease the pressure ofthe permeate 126. If the pressure of the permeate 126 is at thepredetermined value (or within the margin of error), then no action istaken by the controller 160. As explained above, the predetermined valuemay be associated with the requirements of the demand source 150, and insome instances may be equal to the pressure of UPW delivered to thesystem, and in other instances may be equal to the UPW pressure within acertain margin of error, e.g., ±2 psi.

In some embodiments, the water polishing system further includes aholding tank. FIG. 6 is a flow diagram of one embodiment of a waterpolishing system 600 that includes a holding tank 655 that is disposedalong the recirculation conduit 120. Water polishing system 600 issimilar to water polishing system 100 described above in reference toFIG. 4, but includes the holding tank 655, which according to oneembodiment may be positioned at a location along the recirculationconduit 120 that is upstream from the recirculation pump 115, i.e., thesuction side of the recirculation pump 115 and downstream from theblending point 147. The purpose of the holding tank 655 may be to allowfor fluid in the recirculation conduit to accumulate during periods whenthere is little or no consumption demand from the demand source 150.Fluid 170 may accumulate in the holding tank 655. Continuous flowthrough the recirculation conduit 120 is maintained during theseperiods, and the holding tank 655 allows for excess volume of fluid toaccumulate in one location in the recirculation loop.

The holding tank 655 may be a tank or a pipe that is comparable to orgreater than a volume or periodic consumption volume of the demandsource 150. According to one aspect, when the holding tank 655 becomesfull, a majority of the water from the tank continuously passes throughthe UF module 110 during the entire time period that the demand source150 remains idle.

According to an alternative embodiment, the holding tank 655 may bepositioned at the blending point 147 such that it performs the functionof the blending point 147. According to this configuration, the system600 may further include a level control apparatus that functions tomaintain a constant fluid level in the holding tank 655. This permitsUPW from the make-up conduit 107 to match the sum of the consumption ofthe demand source 150 and the retentate 128 exiting via the rejectconduit 124.

The holding tank 655 may also function to lower or maintain a lowertemperature of the permeate 126. As mentioned above, during periodswhere the demand source 150 is not using permeate 126, the permeatetemperature can increase. In addition, the recirculation pump 115 mayincrease the temperature of fluid 170. The ability to allow cooler UPWfrom the source of UPW 105 to be continuously introduced by rejectingretentate 128 at a constant rate helps to maintain a lower temperature,and heat can also be expelled via retentate 128 that exits the system100.

Although not explicitly shown in the accompanying figures, according toone embodiment the water polishing system may further include a catalystmaterial. The catalyst material may be any material that functions toprotect the membrane in filtration unit, such as the UF membrane used inthe UF module 110, by degrading oxidants, such as hydrogen peroxidepresent in the UPW introduced to the UF module. The oxidants may damageor otherwise harm the UF membrane, and removing these materials from thefluid that comes into contact with the membrane can therefore extend theoperating life of the module. The catalyst material may be positioned inbetween the recirculation pump 115 and the inlet 114 of the UF module110 so as to treat the UPW prior to contact with the UF membrane.Non-limiting examples of suitable catalysts include platinum, palladium,or enzymes (e.g., catalase).

The water polishing systems disclosed herein may also include other flowcontrol devices, such as valves, reducers, expanders, and the like. Inaddition, other pressure, temperature, and flow rate sensors may bepositioned at one or more locations throughout the system besides thesensors described herein. Other treatment devices may also be includedin the water polishing system, such as a heat exchanger. For instance, aheat exchanger may be disposed in the recirculation conduit 120 betweenthe recirculation pump 115 and the inlet 114 of the UF module 110. Theheat exchanger may be useful during periods where there is no demandfrom the demand source 150, and the temperature of the permeateincreases.

The water polishing systems described herein may be used in someinstances to retrofit or replace a current POU filtration system. Anexisting facility may be modified to utilize any one or more aspects ofthe water polishing system, and in some instances, the apparatus andmethods may include connecting or configuring an existing facility tocomprise a water polishing system as described herein.

In accordance with certain aspects, a method of facilitating polishingof UPW is provided. The method may facilitation one or more parts of apre-existing or a new treatment system. In certain embodiments, themethod may comprise providing an ultrafiltration (UF) module having aninlet and a permeate outlet and providing instructions to: connect thepermeate outlet to the inlet, connect the permeate outlet to a pressurecontrol valve, and maintain a pressure of permeate exiting the permeateoutlet at a predetermined value with the pressure control valve. In someembodiments, the method of facilitating further includes providinginstructions to connect a recirculation pump to the inlet of the UFmodule, blend the permeate with a source of UPW to form a fluid, anddirect the fluid to an inlet of the recirculation pump. In oneembodiment, the method of facilitating further includes instructions toconnect the permeate outlet to a demand source and measure a pressure ofthe permeate using a pressure sensor prior to blending with the sourceof UPW. In another embodiment, the method of facilitating furtherincludes providing a controller that is configured to be operativelycoupled to the pressure sensor and the pressure control valve. Accordingto a further embodiment, the method of facilitating further includesproviding control instructions to the controller to compare a measuredpressure of the permeate to a predetermined value and, responsive to thecomparison, actuate the pressure control valve. According to at leastone other embodiment, the control instructions instruct the controllerto actuate the pressure control valve to maintain a substantiallyconstant transmembrane pressure (TMP) across a UF membrane of the UFmodule. In yet another embodiment, the method of facilitating furtherincudes providing at least one of the recirculation pump and thepressure control valve.

EXAMPLES

Aspects of the systems and methods described herein may be furtherdemonstrated by the following examples, which are provided for purposesof illustration only, and do not limit the scope of the inventiondescribed herein.

Example 1—Equilibrium Temperature of Permeate Calculations

Calculations were performed to determine the temperature of permeateunder different delivery flow rates. The set-up was assumed to besubstantially similar to that shown in system 100 of FIG. 4, with the UFmodule 110 being a VANOX® POU-F System from Evoqua Water Technologies.The reject flow rate was kept constant at 1.5 gpm while the deliveryflow rate, i.e., permeate flow rate exiting the filter (or permeate flowrate flowing to the demand source 150 in the supply conduit 145), wasvaried between zero and 25 gpm. Two different TMP pressures were usedfor determining the temperature of the permeate. The first used a TMPpressure of 15 psi (labeled as “normal” in FIG. 7) and the second used aTMP pressure of 20 psi (labeled as “maximum” in FIG. 7). The results areshown in the graph of FIG. 7 and are plotted against the UPW supplytemperature, which was 20° C.).

The results indicate that when the permeate delivery flow rate was zerogpm, which is indicative of a situation where the demand source 150 ofFIG. 4 is not consuming any permeate, the temperature of the permeateincreased by 1.2 to 2.0° C. above the incoming UPW supply temperature.This difference decreased with higher delivery flow rates (i.e., insituations where the demand source 150 consumes permeate). At a flowrate of 5 gpm, the temperature of the permeate increased by less than0.5° C., and at a flow rate of 10 gpm, the increase in temperature wasless than 0.25° C. A higher reject flow rate would also cause a lowertemperature increase, i.e., at higher reject flow rates, the temperaturerise would be less. For instance, when the reject flow rate wasincreased to 2 gpm, the temperature increase was 0.9-1.6° C. at thedelivery flow rate of zero gpm.

Example 2—Illustration of Control Philosophy

To help better understand some of the control mechanisms describedabove, three different operational cases are outlined below in referenceto the water polishing system 100 shown in FIG. 4. For purposes ofillustration, the following assumptions are held:

-   1. UPW supply 105 pressure to the system is 80 psi-   2. TMP is 15 psi-   3. For simplicity, the system losses are included in the TMP-   4. Reject flow rate of the retentate 128 is 1.5 gpm

Condition 1—No Demand from Demand Source 150

The recirculation pump 115 delivers fluid at 95 psi (to overcome theTMP) at a flow rate of 31.5 gpm to the UF module 110. The permeate 126will have a pressure of 80 psi and a flow rate of 30 gpm. Since there isno consumption by the demand source 150, the entire 30 gpm of permeate126 may pass through the pressure control valve 125 to be blended with a1.5 gpm flow rate of UPW from the make-up conduit 107 as fluid 170 atthe blending point 147, and is recirculated back to the recirculationpump 115 and UF module 110.

Condition 2—Ramp up from Zero gpm Demand to X gpm Demand from the DemandSource

The recirculation pump 115 continues to deliver fluid at 31.5 gpm at 95psi, but in this instance, there is demand from the demand source 150,so there is a flow of permeate 126 exiting the recirculation conduit 120through the supply conduit 145. The pressure of the permeate 126 at thelocation of the pressure sensor 140 will therefore be less than 80 psi.Based on this measurement, the pressure control valve 125 can beactuated by the controller 160 to close by a certain amount to increasethe pressure of the permeate 126 upstream from the blending point 147back up to 80 psi, so that when the fluid 170 reaches the recirculationpump 115 the pressure is at 80 psi.

Condition 3—Ramp Down from X gpm Demand to Zero gpm Demand from theDemand Source

The recirculation pump 115 still continues to deliver fluid at 31.5 gpmat a pressure of 95 psi, but in this instance, consumption from thedemand source 150 is decreasing. The rate of egress of permeate 126 outof the supply conduit 145 is decreasing, which causes the pressure ofthe permeate 126 at the location of the pressure sensor 140 to begreater than 80 psi. The pressure control valve 125 can therefore beactuated based on the measured pressure value by the controller to openby a certain amount, which decreases the pressure of the permeate 126 atthe location upstream from the blending point 147 back down to 80 psi.Fluid 170 that reaches the recirculation pump 115 will therefore have apressure of 80 psi.

Example 3-Vibration Experiment

An experiment was performed to test how a low level adjustable vibrationsource coupled to a filtration unit would influence the retention ofsmall particles, e.g., 20 nm and less in diameter, by the filtrationunit.

The filtration unit was a UF POU filtration unit. Particle countmeasurements of the effluent were performed using a particle analyzerboth before and after a vibration source was applied to the externalside of the filtration unit. The vibration source was from a vacuum pumpplaced in proximity to the POU filtration unit.

Control data is shown on the left side of FIG. 8 and shows particleconcentration measurements of both 10 nm and 20 nm sized particles takenover an approximate 22 hour time period. This data indicated anapproximate 800,000 cts/ml concentration of 20 nm sized particles, andan approximate 1,000,000 cts/ml concentration of 10 nm sized particles.

The right side of FIG. 8 shows particle measurement concentration datataken with a vibration applied to the POU filtration unit. Measurementswere taken over an approximate 32 hour time period. The concentration ofparticles decreased by three to fivefold, with the concentration of 20nm particles decreasing from about 800,000 cts/ml to about 200,000 andthe concentration of 10 nm particles decreasing from about 1,000,000cts/ml to about 300,000 cts/ml. The results were counterintuitive sincevibration usually functions to loosen particles from the components ofthe filtration system, including the filtration membrane and housing,piping, and other components exposed to the UPW. Although not explicitlyshown, the testing also indicated that the vibration did notsignificantly improve the retention of particles sized greater than 50nm. Applying a source of vibration to the filtration unit may thereforebe helpful in applications where there is a need to enhance the removalof particles sized at 20 nm and smaller.

FIG. 9 is a graph showing vibration data over time obtained from anaccelerometer positioned on the filtration unit while vibration wasapplied. The vibration data includes x, y, and z directional data fromthe accelerometer. FIG. 10 is a graph showing vibration data from adifferent type of filtration unit, with the same vibration sourceapplied in a similar manner as described above. The filtration unit usedin obtaining this data was from a VANOX® System Hot Water Conditioning(HWC) Cart available from Evoqua Water Technologies and included a UFmembrane. The left side of the graph shown in FIG. 10 is vibration dataduring a period of time where vibration was applied, and the right sideof the graph indicates a period where no vibration was applied.

Example 4—Constant Flux Retention Efficiency

As a comparison to the dead-end cartridge filter retention datapresented in FIG. 1, a UF module that included a 6000 MWCO membrane wastested for particle retention. The UF device was operated under aconstant flux, and the results are presented in FIG. 11. The datapresented in the graph of FIG. 11 shows that under constant flux, the UFmembrane was effective at removing particles having a diameter of largerthan 15 nm, and was capable of removing between about 85-95% ofparticles sized at 10 nm.

Having thus described several aspects of at least one example, it is tobe appreciated that various alterations, modifications, and improvementswill readily occur to those skilled in the art. For instance, examplesdisclosed herein may also be used in other contexts. Such alterations,modifications, and improvements are intended to be part of thisdisclosure, and are intended to be within the scope of the examplesdiscussed herein. Accordingly, the foregoing description and drawingsare by way of example only.

What is claimed is:
 1. A water polishing system comprising: a source ofultrapure water (UPW); an ultrafiltration (UF) module having an inletand a permeate outlet; a recirculation conduit communicating thepermeate outlet with the inlet and forming a recirculation loop; arecirculation pump disposed along the recirculation conduit upstreamfrom the inlet of the UF module and fluidly coupled to the source ofUPW; a supply conduit fluidly coupled to the recirculating conduit and ademand source, the supply conduit positioned downstream from thepermeate outlet; and a pressure control valve disposed along therecirculation conduit downstream from the supply conduit and configuredto maintain a pressure of the permeate at a predetermined value.
 2. Thewater polishing system of claim 1, further comprising a pressure sensorpositioned along the recirculation conduit between the supply conduitand the pressure control valve, configured to generate a pressure signalindicative of a pressure of the permeate.
 3. The water polishing systemof claim 2, further comprising a controller operatively coupled to thepressure sensor and configured to control operation of the pressurecontrol valve based on the pressure signal.
 4. The water polishingsystem of claim 3, further comprising: a UPW make-up conduit fluidlycoupled to the source of UPW and the recirculation conduit; and ablending point where the UPW make-up conduit fluidly connects to therecirculation conduit such that the pressure control valve is positionedbetween the blending point and the pressure sensor, and the controlleris configured to operate the pressure control valve such that thepressure of the permeate is maintained at a predetermined pressurevalue.
 5. The water polishing system of claim 4, wherein therecirculation pump is positioned downstream from the blending point suchthat a fluid including UPW and permeate is delivered to therecirculation pump at the predetermined pressure value.
 6. The waterpolishing system of claim 5, wherein the recirculation pump isconfigured to operate at a predetermined constant speed.
 7. The waterpolishing system of claim 5, wherein the UF module further includes areject outlet for retentate from the UF module and a reject conduitfluidly coupled to the reject outlet.
 8. The water polishing system ofclaim 1, further comprising a holding tank disposed along therecirculation conduit.
 9. The water polishing system of claim 1, furthercomprising a vibration source coupled to the UF module.
 10. A method oftreating water, comprising: receiving a flow of ultrapure water (UPW)directed toward a recirculation pump from a source of UPW; setting thespeed of the recirculation pump to a predetermined constant speed;directing a fluid including the UPW to an inlet of an ultrafiltration(UF) module; recirculating at least a portion of permeate from apermeate outlet of the UF module to the inlet using the recirculationpump and a recirculation conduit; providing a valve arrangement actuablefrom a closed to an open condition for effecting pressure of permeate tothe recirculation pump; measuring a pressure of the permeate at aposition that is upstream from the valve arrangement and downstream fromthe permeate outlet; comparing the measured pressure of the permeate toa predetermined value; and responsive to the comparison, actuating thevalve arrangement.
 11. The method of claim 10, wherein the valvearrangement is actuated to maintain a substantially constanttransmembrane pressure (TMP) across a UF membrane of the UF module. 12.The method of claim 10, further comprising combining the flow of UPWwith permeate at a blending point positioned along the recirculationconduit such that the valve arrangement is positioned upstream from theblending point.
 13. The method of claim 12, further comprising providinga supply conduit to a demand source along the recirculation conduit at aposition upstream from the pressure measurement position and downstreamfrom the permeate outlet.
 14. The method of claim 10, wherein therecirculation pump is configured to deliver the fluid to the UF moduleat a pressure that is greater than the predetermined value.
 15. Themethod of claim 10, further comprising applying a vibration to the UFmodule.
 16. A method of facilitating polishing of ultrapure water (UPW),comprising: providing an ultrafiltration (UF) module having an inlet anda permeate outlet; and providing instructions to: connect the permeateoutlet to the inlet; connect the permeate outlet to a pressure controlvalve; and maintain a pressure of permeate exiting the permeate outletat a predetermined value with the pressure control valve.
 17. The methodof claim 16, further comprising providing instructions to: connect arecirculation pump to the inlet of the UF module; blend the permeatewith a source of UPW to form a fluid; and direct the fluid to an inletof the recirculation pump.
 18. The method of claim 17, furthercomprising instructions to: connect the permeate outlet to a demandsource; and measure a pressure of the permeate using a pressure sensorprior to blending with the source of UPW.
 19. The method of claim 18,further comprising providing a controller that is configured to beoperatively coupled to the pressure sensor and the pressure controlvalve.
 20. The method of claim 19, further comprising providing controlinstructions to the controller to compare a measured pressure of thepermeate to a predetermined value and, responsive to the comparison,actuate the pressure control valve.
 21. The method of claim 20, whereinthe control instructions instruct the controller to actuate the pressurecontrol valve to maintain a substantially constant transmembranepressure (TMP) across a UF membrane of the UF module.
 22. The method ofclaim 17, further comprising providing at least one of the recirculationpump and the pressure control valve.